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.\" rtc.4
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.\" Copyright 2002 Urs Thuermann (urs@isnogud.escape.de)
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.\"
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.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%LICENSE_END
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.\"
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.\" $Id: rtc.4,v 1.4 2005/12/05 17:19:49 urs Exp $
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.\"
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.\" 2006-02-08 Various additions by mtk
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.\" 2006-11-26 cleanup, cover the generic rtc framework; David Brownell
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.\"
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.TH RTC 4 2017-09-15 "Linux" "Linux Programmer's Manual"
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.SH NAME
|
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rtc \- real-time clock
|
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.SH SYNOPSIS
|
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#include <linux/rtc.h>
|
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.PP
|
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.BI "int ioctl(" fd ", RTC_" request ", " param ");"
|
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.SH DESCRIPTION
|
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This is the interface to drivers for real-time clocks (RTCs).
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.PP
|
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Most computers have one or more hardware clocks which record the
|
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current "wall clock" time.
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These are called "Real Time Clocks" (RTCs).
|
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One of these usually has battery backup power so that it tracks the time
|
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even while the computer is turned off.
|
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RTCs often provide alarms and other interrupts.
|
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.PP
|
|
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All i386 PCs, and ACPI-based systems, have an RTC that is compatible with
|
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the Motorola MC146818 chip on the original PC/AT.
|
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Today such an RTC is usually integrated into the mainboard's chipset
|
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(south bridge), and uses a replaceable coin-sized backup battery.
|
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.PP
|
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Non-PC systems, such as embedded systems built around system-on-chip
|
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processors, use other implementations.
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They usually won't offer the same functionality as the RTC from a PC/AT.
|
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.SS RTC vs system clock
|
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RTCs should not be confused with the system clock, which is
|
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a software clock maintained by the kernel and used to implement
|
|
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.BR gettimeofday (2)
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and
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.BR time (2),
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as well as setting timestamps on files, and so on.
|
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The system clock reports seconds and microseconds since a start point,
|
|
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defined to be the POSIX Epoch: 1970-01-01 00:00:00 +0000 (UTC).
|
|
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(One common implementation counts timer interrupts, once
|
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per "jiffy", at a frequency of 100, 250, or 1000 Hz.)
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That is, it is supposed to report wall clock time, which RTCs also do.
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.PP
|
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A key difference between an RTC and the system clock is that RTCs
|
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run even when the system is in a low power state (including "off"),
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and the system clock can't.
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Until it is initialized, the system clock can only report time since
|
|
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system boot ... not since the POSIX Epoch.
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So at boot time, and after resuming from a system low power state, the
|
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system clock will often be set to the current wall clock time using an RTC.
|
|
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Systems without an RTC need to set the system clock using another clock,
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maybe across the network or by entering that data manually.
|
|
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.SS RTC functionality
|
|
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RTCs can be read and written with
|
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.BR hwclock (8),
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or directly with the ioctl requests listed below.
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.PP
|
|
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Besides tracking the date and time, many RTCs can also generate
|
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interrupts
|
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.IP * 3
|
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on every clock update (i.e., once per second);
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.IP *
|
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at periodic intervals with a frequency that can be set to
|
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any power-of-2 multiple in the range 2 Hz to 8192 Hz;
|
|
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.IP *
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on reaching a previously specified alarm time.
|
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.PP
|
|
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Each of those interrupt sources can be enabled or disabled separately.
|
|
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On many systems, the alarm interrupt can be configured as a system wakeup
|
|
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event, which can resume the system from a low power state such as
|
|
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Suspend-to-RAM (STR, called S3 in ACPI systems),
|
|
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Hibernation (called S4 in ACPI systems),
|
|
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or even "off" (called S5 in ACPI systems).
|
|
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On some systems, the battery backed RTC can't issue
|
|
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interrupts, but another one can.
|
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.PP
|
|
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The
|
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.I /dev/rtc
|
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(or
|
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.IR /dev/rtc0 ,
|
|
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.IR /dev/rtc1 ,
|
|
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etc.)
|
|
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device can be opened only once (until it is closed) and it is read-only.
|
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On
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.BR read (2)
|
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and
|
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.BR select (2)
|
|
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the calling process is blocked until the next interrupt from that RTC
|
|
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is received.
|
|
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Following the interrupt, the process can read a long integer, of which
|
|
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the least significant byte contains a bit mask encoding
|
|
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the types of interrupt that occurred,
|
|
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while the remaining 3 bytes contain the number of interrupts since the
|
|
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last
|
|
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.BR read (2).
|
|
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.SS ioctl(2) interface
|
|
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The following
|
|
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.BR ioctl (2)
|
|
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requests are defined on file descriptors connected to RTC devices:
|
|
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.TP
|
|
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.B RTC_RD_TIME
|
|
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Returns this RTC's time in the following structure:
|
|
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.IP
|
|
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.in +4n
|
|
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.EX
|
|
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struct rtc_time {
|
|
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int tm_sec;
|
|
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int tm_min;
|
|
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int tm_hour;
|
|
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int tm_mday;
|
|
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int tm_mon;
|
|
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int tm_year;
|
|
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int tm_wday; /* unused */
|
|
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int tm_yday; /* unused */
|
|
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int tm_isdst; /* unused */
|
|
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};
|
|
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.EE
|
|
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.in
|
|
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.IP
|
|
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The fields in this structure have the same meaning and ranges as for the
|
|
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.I tm
|
|
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structure described in
|
|
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.BR gmtime (3).
|
|
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A pointer to this structure should be passed as the third
|
|
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.BR ioctl (2)
|
|
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argument.
|
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.TP
|
|
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.B RTC_SET_TIME
|
|
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Sets this RTC's time to the time specified by the
|
|
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.I rtc_time
|
|
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structure pointed to by the third
|
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.BR ioctl (2)
|
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argument.
|
|
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To set the
|
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RTC's time the process must be privileged (i.e., have the
|
|
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.B CAP_SYS_TIME
|
|
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capability).
|
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.TP
|
|
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.BR RTC_ALM_READ ", " RTC_ALM_SET
|
|
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Read and set the alarm time, for RTCs that support alarms.
|
|
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The alarm interrupt must be separately enabled or disabled using the
|
|
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.BR RTC_AIE_ON ", " RTC_AIE_OFF
|
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requests.
|
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The third
|
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.BR ioctl (2)
|
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argument is a pointer to an
|
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.I rtc_time
|
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structure.
|
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Only the
|
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.IR tm_sec ,
|
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.IR tm_min ,
|
|
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and
|
|
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.I tm_hour
|
|
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fields of this structure are used.
|
|
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.TP
|
|
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.BR RTC_IRQP_READ ", " RTC_IRQP_SET
|
|
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Read and set the frequency for periodic interrupts,
|
|
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7cfc04 |
for RTCs that support periodic interrupts.
|
|
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The periodic interrupt must be separately enabled or disabled using the
|
|
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.BR RTC_PIE_ON ", " RTC_PIE_OFF
|
|
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requests.
|
|
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The third
|
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.BR ioctl (2)
|
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argument is an
|
|
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.I "unsigned long\ *"
|
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or an
|
|
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.IR "unsigned long" ,
|
|
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respectively.
|
|
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The value is the frequency in interrupts per second.
|
|
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The set of allowable frequencies is the multiples of two
|
|
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7cfc04 |
in the range 2 to 8192.
|
|
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7cfc04 |
Only a privileged process (i.e., one having the
|
|
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.B CAP_SYS_RESOURCE
|
|
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capability) can set frequencies above the value specified in
|
|
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7cfc04 |
.IR /proc/sys/dev/rtc/max-user-freq .
|
|
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(This file contains the value 64 by default.)
|
|
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7cfc04 |
.TP
|
|
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7cfc04 |
.BR RTC_AIE_ON ", " RTC_AIE_OFF
|
|
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Enable or disable the alarm interrupt, for RTCs that support alarms.
|
|
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7cfc04 |
The third
|
|
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.BR ioctl (2)
|
|
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argument is ignored.
|
|
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7cfc04 |
.TP
|
|
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7cfc04 |
.BR RTC_UIE_ON ", " RTC_UIE_OFF
|
|
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7cfc04 |
Enable or disable the interrupt on every clock update,
|
|
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7cfc04 |
for RTCs that support this once-per-second interrupt.
|
|
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7cfc04 |
The third
|
|
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.BR ioctl (2)
|
|
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argument is ignored.
|
|
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7cfc04 |
.TP
|
|
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7cfc04 |
.BR RTC_PIE_ON ", " RTC_PIE_OFF
|
|
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7cfc04 |
Enable or disable the periodic interrupt,
|
|
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7cfc04 |
for RTCs that support these periodic interrupts.
|
|
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7cfc04 |
The third
|
|
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7cfc04 |
.BR ioctl (2)
|
|
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7cfc04 |
argument is ignored.
|
|
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7cfc04 |
Only a privileged process (i.e., one having the
|
|
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7cfc04 |
.B CAP_SYS_RESOURCE
|
|
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7cfc04 |
capability) can enable the periodic interrupt if the frequency is
|
|
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7cfc04 |
currently set above the value specified in
|
|
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7cfc04 |
.IR /proc/sys/dev/rtc/max-user-freq .
|
|
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7cfc04 |
.TP
|
|
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7cfc04 |
.BR RTC_EPOCH_READ ", " RTC_EPOCH_SET
|
|
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7cfc04 |
Many RTCs encode the year in an 8-bit register which is either
|
|
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7cfc04 |
interpreted as an 8-bit binary number or as a BCD number.
|
|
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7cfc04 |
In both cases,
|
|
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the number is interpreted relative to this RTC's Epoch.
|
|
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7cfc04 |
The RTC's Epoch is
|
|
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7cfc04 |
initialized to 1900 on most systems but on Alpha and MIPS it might
|
|
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7cfc04 |
also be initialized to 1952, 1980, or 2000, depending on the value of
|
|
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7cfc04 |
an RTC register for the year.
|
|
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7cfc04 |
With some RTCs,
|
|
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7cfc04 |
these operations can be used to read or to set the RTC's Epoch,
|
|
Packit |
7cfc04 |
respectively.
|
|
Packit |
7cfc04 |
The third
|
|
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7cfc04 |
.BR ioctl (2)
|
|
Packit |
7cfc04 |
argument is an
|
|
Packit |
7cfc04 |
.I "unsigned long\ *"
|
|
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7cfc04 |
or an
|
|
Packit |
7cfc04 |
.IR "unsigned long" ,
|
|
Packit |
7cfc04 |
respectively, and the value returned (or assigned) is the Epoch.
|
|
Packit |
7cfc04 |
To set the RTC's Epoch the process must be privileged (i.e., have the
|
|
Packit |
7cfc04 |
.B CAP_SYS_TIME
|
|
Packit |
7cfc04 |
capability).
|
|
Packit |
7cfc04 |
.TP
|
|
Packit |
7cfc04 |
.BR RTC_WKALM_RD ", " RTC_WKALM_SET
|
|
Packit |
7cfc04 |
Some RTCs support a more powerful alarm interface, using these ioctls
|
|
Packit |
7cfc04 |
to read or write the RTC's alarm time (respectively) with this structure:
|
|
Packit |
7cfc04 |
.PP
|
|
Packit |
7cfc04 |
.RS
|
|
Packit |
7cfc04 |
.in +4n
|
|
Packit |
7cfc04 |
.EX
|
|
Packit |
7cfc04 |
struct rtc_wkalrm {
|
|
Packit |
7cfc04 |
unsigned char enabled;
|
|
Packit |
7cfc04 |
unsigned char pending;
|
|
Packit |
7cfc04 |
struct rtc_time time;
|
|
Packit |
7cfc04 |
};
|
|
Packit |
7cfc04 |
.EE
|
|
Packit |
7cfc04 |
.in
|
|
Packit |
7cfc04 |
.RE
|
|
Packit |
7cfc04 |
.IP
|
|
Packit |
7cfc04 |
The
|
|
Packit |
7cfc04 |
.I enabled
|
|
Packit |
7cfc04 |
flag is used to enable or disable the alarm interrupt,
|
|
Packit |
7cfc04 |
or to read its current status; when using these calls,
|
|
Packit |
7cfc04 |
.BR RTC_AIE_ON " and " RTC_AIE_OFF
|
|
Packit |
7cfc04 |
are not used.
|
|
Packit |
7cfc04 |
The
|
|
Packit |
7cfc04 |
.I pending
|
|
Packit |
7cfc04 |
flag is used by
|
|
Packit |
7cfc04 |
.B RTC_WKALM_RD
|
|
Packit |
7cfc04 |
to report a pending interrupt
|
|
Packit |
7cfc04 |
(so it's mostly useless on Linux, except when talking
|
|
Packit |
7cfc04 |
to the RTC managed by EFI firmware).
|
|
Packit |
7cfc04 |
The
|
|
Packit |
7cfc04 |
.I time
|
|
Packit |
7cfc04 |
field is as used with
|
|
Packit |
7cfc04 |
.B RTC_ALM_READ
|
|
Packit |
7cfc04 |
and
|
|
Packit |
7cfc04 |
.B RTC_ALM_SET
|
|
Packit |
7cfc04 |
except that the
|
|
Packit |
7cfc04 |
.IR tm_mday ,
|
|
Packit |
7cfc04 |
.IR tm_mon ,
|
|
Packit |
7cfc04 |
and
|
|
Packit |
7cfc04 |
.I tm_year
|
|
Packit |
7cfc04 |
fields are also valid.
|
|
Packit |
7cfc04 |
A pointer to this structure should be passed as the third
|
|
Packit |
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.BR ioctl (2)
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argument.
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.SH FILES
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.TP
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.IR /dev/rtc ", " /dev/rtc0 ", " /dev/rtc1 ", etc."
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RTC special character device files.
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.TP
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.IR /proc/driver/rtc
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status of the (first) RTC.
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.SH NOTES
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When the kernel's system time is synchronized with an external
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reference using
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.BR adjtimex (2)
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it will update a designated RTC periodically every 11 minutes.
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To do so, the kernel has to briefly turn off periodic interrupts;
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this might affect programs using that RTC.
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.PP
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An RTC's Epoch has nothing to do with the POSIX Epoch which is
|
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used only for the system clock.
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.PP
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|
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If the year according to the RTC's Epoch and the year register is
|
|
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less than 1970 it is assumed to be 100 years later, that is, between 2000
|
|
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and 2069.
|
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.PP
|
|
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Some RTCs support "wildcard" values in alarm fields, to support
|
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scenarios like periodic alarms at fifteen minutes after every hour,
|
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or on the first day of each month.
|
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Such usage is nonportable;
|
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portable user-space code expects only a single alarm interrupt, and
|
|
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will either disable or reinitialize the alarm after receiving it.
|
|
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.PP
|
|
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Some RTCs support periodic interrupts with periods that are multiples
|
|
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of a second rather than fractions of a second;
|
|
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multiple alarms;
|
|
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programmable output clock signals;
|
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nonvolatile memory;
|
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and other hardware
|
|
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capabilities that are not currently exposed by this API.
|
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.SH SEE ALSO
|
|
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.BR date (1),
|
|
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.BR adjtimex (2),
|
|
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.BR gettimeofday (2),
|
|
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.BR settimeofday (2),
|
|
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.BR stime (2),
|
|
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.BR time (2),
|
|
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.BR gmtime (3),
|
|
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.BR time (7),
|
|
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.BR hwclock (8)
|
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.PP
|
|
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.I Documentation/rtc.txt
|
|
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in the Linux kernel source tree
|
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.SH COLOPHON
|
|
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This page is part of release 4.15 of the Linux
|
|
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.I man-pages
|
|
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project.
|
|
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A description of the project,
|
|
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information about reporting bugs,
|
|
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and the latest version of this page,
|
|
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can be found at
|
|
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\%https://www.kernel.org/doc/man\-pages/.
|